Eur. J. Entomol. 105: 513–520, 2008 http://www.eje.cz/scripts/viewabstract.php?abstract=1359 ISSN 1210-5759 (print), 1802-8829 (online) Corn leaf aphid, Rhopalosiphum maidis (Hemiptera: Aphididae), is a key to greenbug, Schizaphis graminum (Hemiptera: Aphididae), biological control in grain sorghum, Sorghum bicolor GERALD J. MICHELS, JR. and JAMES H. MATIS Texas Agricultural Experiment Station, P. O Drawer 10, 2301 Experiment Station Road, Bushland, TX 79012, USA; e-mail: [email protected] Department of Statistics, Texas A&M University, College Station, TX 77843-3143, USA; e-mail: [email protected] Key words. Corn leaf aphid, greenbug, coccinellids, biological control, grain sorghum, pest management Abstract. In the sorghum/aphid/ladybeetle ecosystem found in the Texas High Plains Region of the United States, we found that the corn leaf aphid, Rhopalosiphum maidis (Fitch), is a key aphid species that provides a critical early-season food source for native coc- cinellids. From 1988 to 2000 data on the seasonal abundance of sorghum-infesting aphids and ladybeetles were collected from a total of 21 irrigated and 12 rain-fed grain sorghum fields. The data indicated that successful biological control of the greenbug by cocci- nellids is normally dependent on early-season colonization of the sorghum field by corn leaf aphids. When corn leaf aphids exceeded 100/plant before sorghum boot stage greenbugs never exceeded 125 aphids/plant. In all cases where greenbugs were found in densi- ties that would cause economic damage to sorghum (>250/plant), corn leaf aphids reached a density of 100 or more per plant after sorghum reached the boot stage. In irrigated fields, the first record of coccinellid eggs and peak coccinellid abundance were posi- tively and significantly (p = 0.05) correlated to the day of the year when corn leaf aphids reached or exceeded a minimum of 100/plant and corn leaf aphid peak abundance in both irrigated and rainfed fields. On the other hand, greenbug peak abundance was significantly correlated only to coccinellid peak abundance in irrigated fields. Regression analyses indicated that in paired analyses of irrigated and rain-fed sorghum fields, an increase of one aphid at time t, resulted in an increase in coccinellid peak abundance from 0.024 to 0.025 per 15 m of row at time t + 2 depending on aphid species, if corn leaf aphids reached a level of 100 or more per plant by sorghum boot stage and irrigation parameters. We concluded that corn leaf aphids are an important early-season food source for predaceous coccinellids, drawing these predators into the fields where they feed on the aphids and deposit eggs, engendering a captive larval population that is present when greenbug first begin to enter the field later in the season. INTRODUCTION rarely cause economic loss and can be considered helpful At times, pest management practices can concentrate by attracting coccinellids. Cate et al. (1973) and Wilde & too narrowly on a specific predator prey interaction Ohiagu (1976) both concluded that using insecticides to without bringing into account other factors. The control corn leaf aphids did not increase sorghum yield greenbug, Schizaphis graminum (Rondani), is a serious nor did corn leaf aphid infestations caused significant economic pest of grain sorghum, Sorghum bicolor (L.) damage to grain sorghum. In addition, applying insecti- Moench, in the United States Great Plains. Existing bio- cides for corn leaf aphids disrupts or eliminates predators control agents, primarily coccinellids in the genus Hippo- and parasitoids which attack greenbugs in sorghum or damia, can keep this pest at sub-economic levels if there migrate and aid in biological control of aphids in crops is a proper understanding of the ecological interactions such as cotton (Cronholm et al., 1998). among the native predaceous coccinellids and their aphid Research by Kring et al. (1985), Kring & Gilstrap prey. (1986), and Rice & Wilde (1988) clearly indicated a rela- In the early 1980s in the High Plains of Texas, aphids tionship between corn leaf aphids and predaceous cocci- infesting grain sorghum, Sorghum bicolor (L.) Moench, nellids which was a key to greenbug, Schizaphis regardless of species, were controlled by aerially-applied graminum (Rondani), biological control. This research insecticides. Timing for the applications was based on prompted us to examine the interactions among the aphids seeing “slick leaves” caused by the deposition of corn leaf and predators over an extended period of time and by irri- aphid, Rhopalosiphum maidis (Fitch), honeydew. This gated and rain-fed agronomic practices, to determine if was an unsophisticated control strategy which typically biological control of greenbugs in grain sorghum could be required multiple insecticide applications for aphids and better understood and enhanced by conserving natural oftentimes resulted in late-season infestations by Banks enemies. This paper presents the results of a multiple-year grass mites, Oligonychus pratensis Banks, for which there study of the seasonal abundance of sorghum-feeding were no effective control measures (Buschman & DePew, aphids and the associated predacious coccinellids. 1990). Cronholm et al. (1998) note that corn leaf aphids 513 MATERIAL AND METHODS 1. To obtain accurate aphid counts, especially corn leaf aphids which are often found deep in the whorl, destructive sampling Field data collection was necessary, From 1988 to 2000, with the exception of 1992, 33 “field- 2. Time and personnel considerations, years” of data on aphids, predators and parasitoids were col- 3. Removing 600 plants per week from a particular field for lected in irrigated and rain-fed grain sorghum fields. A list of destructive sampling during the season could have adverse the aphid, predator and parasitoid species collected during the effects on the distribution of aphid and beneficial insects. research is found in Table 1. A “field year” is defined as one The same sampling routine was carried out each year. We field sampled throughout the growing season. In Bushland, began observing fields in early June and sampling started when Potter Co., TX, data were collected for 12 years in irrigated and corn leaf aphids first began to appear in the field and continued four years in rain-fed grain sorghum fields. In Etter, Moore Co., on a weekly basis until greenbug densities dropped to zero for TX, data were collected for seven years in irrigated and six two consecutive weeks. At times, heavy rains prevented sam- years in rain-fed grain sorghum fields. Two additional fields pling on a strict seven-day interval. However, samples were were sampled in Oldham Co. (irrigated) and in Gray Co., TX, taken as closely as possible to the planned sampling date (Table (rain-fed) in 1994 and two fields were sampled in Parmer Co., 2). TX, (one irrigated and one rain-fed) in 1995. Standard agro- Analyses of parasitoids in relation to initiating greenbug bio- nomic practices for grain sorghum production were followed logical control were omitted from this paper. Although parasi- each year. toids are important to the overall seasonal abundance of green- Fifty consecutive plants down a field row (approx. 15 m) bugs, and to a lesser extent, corn leaf aphids, in this region, from a random starting point were examined by hand. Predator parasitoids occur late in the season and perform more of a and parasitoid numbers/plant were recorded on standardized “clean-up” function rather than being critical to suppression of data sheets. After completing predator and parasitoid sampling, greenbugs when they can most damage gain sorghum. 12 plants were randomly selected from within the 50-plant sam- ple, cut off at the base, and all corn leaf aphids and greenbugs Data analyses were counted. The process was repeated 12 times at each sam- Pearson’s Correlation Coefficients comparing corn leaf aphid pling date, giving a total of 600 plants sampled for predators and peak abundance, the date when corn leaf aphids averaged parasitoids and 144 plants sampled for aphids. 100/plant, and greenbug peak abundance to the occurrence of Different numbers of plants were sampled for natural enemies the first predacious coccinellid eggs and the peak coccinellid and aphids for three reasons: predator abundance were determined using SAS ver. 8.0 (1999) PROC CORR. Regression analyses were performed using SPSS TABLE 1. Aphid, predator and parasitoid species collected in grain sorghum fields in the Texas High Plains from 1988–2000. Family Percentage Percentage of Total recorded Genus and species of all samples 1 Coccinellidae all predators parasitoids aphids Aphididae Rhopalosiphum maidis (Fitch) 297,363 43.21 53.19 Schizaphis graminum (Rondani) 261,721 57.15 46.81 Coccinellidae (adults) Hippodamia convergens Guérin-Ménéville 18,652 56.96 61.46 57.05 Hippodamia sinuata Mulsant 9,962 36.58 32.82 30.47 Other 996 12.71 3.28 3.05 Scymnus (Pullus) loweii 342 5.26 1.13 1.05 Coleomegilla maculata Timberlake 302 4.05 1.00 0.92 Coccinella septempunctata L. 60 2.37 0.20 0.18 Hippodamia parenthesis (Say) 24 1.75 0.08 0.07 Olla v-nigrum (Mulsant) 12 0.26 0.04 0.04 Coccinellidae egg masses 1,689 Coccinellidae larvae 14,842 Coccinellidae pupae 4,170 Chrysopidae Chrysoperla sp. larvae 1,103 40.15 3.37 Chrysoperla sp. eggs 9,775 Nabidae Nabis americoferus Carayon 1,205 8.57 3.68 Syrphidae (larvae) 39 4.94 0.12 Aphelinidae Aphelinus varipes (Foerster) mummies 19,590 11.03 19.48 Braconidae Lysiphlebus testaceipes (Cresson) mummies 80,972 19.34 80.52 1 Percentage